9/29/09 D-13
Capacitor (Condenser): A microphone that uses the motion
of a thin diaphragm caused by the sound to change the
capacitance of an electrical circuit and thereby to create a
signal. For high sensitivity, this device has a voltage applied
across the diaphragm from an internal source.
Electret: A microphone that uses the motion of a thin
diaphragm caused by the sound to change the capacitance of
an electrical circuit and thereby to create a signal. The
voltage across the diaphragm is caused by the charge
embedded in the electret material so no internal source is
needed.
Microphone - Uses: The frequency response of
microphones can be adjusted to be used in specific
applications. Among those used are:
Frontal incidence (Free Field): The microphone has been
adjusted to have an essentially flat frequency response when
in a space relatively free of reflections and when pointed at
the source of the sound.
Random incidence: The microphone has been adjusted to
have an essentially flat frequency response for sound waves
impinging on the microphone from all directions.
Pressure: The microphone has not been adjusted to have an
essentially flat frequency response for sound waves
impinging on the microphone from all directions.
What a microphone measures: A microphone detects more
than just sound
. The motion of a microphone diaphragm is
in response to a force acting on it. The force can be caused
by a number of sources only one of which are we interested:
sound. Non-sound forces are: (1) direct physical contact
such as that with a finger or a raindrop; (2) those caused by
the movement of air over the diaphragm such as
environmental wind or blowing; (3) those caused by
vibration of the microphone housing; and (4) those caused
by strong electrostatic fields.
Rules:
1. Do not permit any solid or liquid to touch the microphone
diaphragm. Keep a protective grid over the diaphragm.
2. Do not blow on a microphone and use a wind screen over
the microphone to reduce the effect of wind noise.
3. Mount microphones so their body is not subject to
vibration, particularly in direction at right angles to the plane
of the diaphragm.
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